Introduce KAUTH_REQ_NETWORK_SOCKET_OPEN, to check if opening a socket is
allowed. It takes three int * arguments indicating domain, type, and
protocol. Replace previous KAUTH_REQ_NETWORK_SOCKET_RAWSOCK with it (but
keep it still).
Places that used to explicitly check for privileged context now don't
need it anymore, so I replaced these with XXX comment indiacting it for
future reference.
Documented and updated examples as well.